2- Use string service tiers in session protocol (#20971)

## Summary
- break service tier session/op/app-server protocol fields from the
closed enum to string tier ids
- send the service tier string directly through model requests, prewarm,
compaction, memories, and TUI/app-server turn starts
- regenerate app-server protocol JSON/TypeScript schemas, removing the
standalone ServiceTier TS enum

## Verification
- just fmt
- cargo check -p codex-core -p codex-app-server -p codex-tui
- just write-app-server-schema

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/session_state.rs

@@ -25,7 +25,7 @@ pub(crate) struct ThreadSessionState {
     pub(crate) thread_name: Option<String>,
     pub(crate) model: String,
     pub(crate) model_provider_id: String,
-    pub(crate) service_tier: Option<codex_protocol::config_types::ServiceTier>,
+    pub(crate) service_tier: Option<String>,
     pub(crate) approval_policy: AskForApproval,
     pub(crate) approvals_reviewer: codex_protocol::config_types::ApprovalsReviewer,
     /// Canonical active permissions for this session. Legacy app-server